A sequence homologous to the mammalian "oncogene", p53 was detected in fish showing great evolutionary conservation. During the embryonal development of the chicken the decrease of this nuclear phosphoprotein was traced to a posttranscriptional step in the mRNA processing, eventually accounting for the decline in the steady state level of the protein. Similar declines in p53 mRNA were found in two different types of induced differentiation in culture of the rabbit tracheal epithelial cells. This together with earlier experiments on retinoic acid induced differentiation of embryonal carcinoma cells, indicates that the decline of p53 mRNA is a common correlate of the cellular differentiation processes. Upon stimulation of adrenergic receptors with isoprotenerol a very rapid and great increase in p53 (and also of c-fos and c-myc) mRNA was observed in rat parotid acinar cells. Numerous SV40 transformed murine and human cell lines were found in which a stable p53 is not in complex with the T antigen or with any other protein, demonstrating that other, yet unknown mechanisms can result in the stability and thus elevated level of the p53 protein. In spontaneous transformation of mouse cells the elevated level of p53 and its half life was unrelated to the cellular tumorigenicity as it was the mRNA levels of p53 and also all the major hitherto recognized proto-oncogenes. However, in the spontaneous transformation a very significant (10/10) correlation was recognized between spontaneous tumorigenecity and a specific change in the heparan sulfate structure. The same heparan sulfate change was recognized before in SV40 transformation of cells. The clonal analysis of the cell systems and the method of selection we present for variant cells for their ability to colonize in the host could be invaluable by allowing a systematic analysis of the natural evolution of tumors.